This invention relates to the determination of engine exhaust backpressure and more particularly to a method and means for remotely determining the average backpressure at a cylinder exhaust port in a crankcase scavenged, two-stroke engine by averaging the pressure of air within a crankcase chamber during a portion of the engine operating cycle. A crankcase scavenged two-stroke engine has a separate crankcase chamber for each engine cylinder. During portions of the engine operating cycle, air is inducted into each crankcase chamber, compressed during the time when the crankcase chamber is decreasing in volume, and then transferred to the associated cylinder combustion chamber where it is mixed with fuel for ignition.
In a direct fuel injected, crankcase scavenged, two-stroke engine, knowledge of the mass of air per cylinder available for combustion is required for the precise control of engine operating parameters such as spark advance, injection timing, and fueling requirements. Conventionally, the mass of air per cylinder inducted into the engine is measured during an engine cycle and then compensated by a factor known as the cylinder trapping efficiency to determine the mass of air actually captured within a cylinder. The trapping efficiency is a function of engine design, engine rotational speed, the mass of air per cylinder inducted into the engine, and the exhaust backpressure.
In a crankcase scavenged, two-stroke engine, the exhaust backpressure is especially significant with regard to the trapping efficiency because of the absence of engine valves. Instead of valves, this type of engine has inlet and exhaust ports opening into each cylinder wall. During the downstroke of a piston in a cylinder, the exhaust port is first uncovered by the moving piston to release combustion products, followed by the uncovering of the inlet port to enable the entry of a fresh charge of air for the next combustion event. Because there is significant overlap in the in the periods of time during which the inlet and exhaust ports are open, a portion of the fresh air charge escapes out the open exhaust port, and will not be available for the next combustion event. The larger the backpressure appearing at the exhaust port, the smaller will be the amount of air which escapes, due to the reduced pressure differential between the cylinder and the engine exhaust system. Also, as the backpressure increases, the resistance to airflow from the crankcase into the cylinder increases, thereby reducing the mass of air transferred.
Since the average exhaust backpressure is equal to the local barometric pressure plus the pressure drop associated with the engine exhaust system, changes in the exhaust system, in the altitude of engine operation, or in local barometric pressure will affect the amount of fresh air which is available for combustion within a cylinder For the above reasons, some measure of engine exhaust backpressure, either direct or indirect, is essential for proper engine control.
A pressure sensor can be used to directly measure the exhaust backpressure within the exhaust system, however, such a sensor will generally be expensive and have poor durability due to the hostility of the exhaust system environment. Also, an average value for the exhaust backpressure, near the engine exhaust port, can be obtained indirectly by measuring barometric pressure, and then adding an estimated pressure drop associated with the engine exhaust system. The major disadvantage with this scheme is inaccuracy due to variations in the actual pressure drop across the exhaust system caused by exhaust gas heating, changes in engine speed, and physical changes in the exhaust system with time.
Consequently, an alternative, more accurate and dependable technique is needed for determining the exhaust backpressure of a crank case scavenged two-stroke engine for use in engine control compensation.